In vacuum circuit interrupters and contactors, for example of the type taught in U.S. Pat. No. 4,109,123 to Lipperts (incorporated herein by reference), the interruption of current typically occurs at the end of the half cycle in which the contacts are separated. When this interruption occurs, the arc current becomes unstable just before the actual sinusoidal zero of current, and drops suddenly to zero. This abrupt drop can cause high voltage transients to be generated in the associated circuit inductance. These transients have a magnitude related to the instantaneous currrent prior to the sinusoidal zero at which instability occurred and can cause electrical breakdown in equipment used in the interrupted circuit. This problem of arc instability in vacuum interrupters has typically been addressed either through the use of surge suppressors, or through the use of a contact material for which the arc is stable to very small currents prior to the natural current zero. The first solution is very expensive. The latter solution compromises operation of the vacuum interrupter in that electrode materials having stable arcing characteristics, typically soft metals with low melting temperatures such as silver, are known to have poor dielectric capabilities. It would thus be desirable to provide a contact assembly for use in a vacuum interrupter which provides the interrupter with both stable arcing characteristics and a high dielectric strength.
In typical air circuit breakers, for example of tthe type taught in U.S. Pat. No. 4,513,268 to Raymond K. Seymour et al. (incorporated herein by reference and assigned to the assignee of the present invention), contacts are constructed of an expensive silver-tungsten alloy. The silver is generally used to provide low contact resistance during switching, and the alloy is used as a sponge to hold molten silver during high level arcing short circuit conditions. In many applications, the size of the contacts must be very large to dissipate heat, and the cost of the contacts becomes prohibitive. However, it is not sufficient merely to cut back on the silver content of the contacts, as the silver provides the low contact resistance. Accordingly, it would be desirable to provide contacts for use in air circuit breakers which provide low contact resistance and are inexpensive to manufacture. It would be further desirable if such contacts acted to stabilize any arc drawn therebetween when the air circuit breakers opened.
The Lipperts patent, cited hereinabove, shows a contact for use in a vacuum interrupter, the contact comprising an internal hollow in the form of a vertically disposed spiral and horizontally disposed cap overlying the spiral, this cap and spiral being filled with an insulating material. It would be desirable to provide a contact assembly for primary use in a vacuum interrupter which provides better arc stabilizing characteristics than that shown in Lipperts. It would be further desirable to provide a contact assembly for primary use in an air circuit breaker which uses inexpensive materials, provides stable arcing conditions, and which does not interfere with the main current flow through the closed contacts as does the cap shown in Lipperts.